When manipulating large images for backgrounds or full-screen image galleries, one of the most common problems that I've encountered is maintaining the image's aspect ratio on resize. Especially so if I'm playing with a dynamically fed gallery that contains images of different size ratios. So here's the solution to your woes. It's one that's worth saving to one side as a "copy and paste solution". Increase the height of the image (or movieclip) to the size that you want it to be and then apply the following line of script...
mc.scaleX > mc.scaleY ? mc.scaleY = mc.scaleX : mc.scaleX = mc.scaleY;
mc._xscale > mc._yscale ? mc._yscale = mc._xscale : mc._xscale = mc._yscale;
("mc" of course should be replaced with the target of your image or movieclip)
Hope that helps!